1. Field of the Invention
The present invention relates to an image forming apparatus of a cleanerless process in which a toner image is formed on an image bearing body by a developing device and a toner remaining on the image bearing body is collected by the developing device, and particularly to an improvement in picture quality and an increase in efficiency of toner recycle.
2. Description of the Related Art
Hitherto, there is known an image forming apparatus adopting an intermediate transfer system in which toner images of four colors are transferred onto an intermediate transfer body, and the toner images of the four colors transferred on the intermediate transfer body are collectively transferred onto a transfer member such as a sheet.
Among the intermediate transfer system image forming apparatuses as stated above, there is known one adopting a so-called four-rotation intermediate transfer system in which toner images are formed on one image bearing body by plural developing devices disposed in the circumferential direction, the transfer of the toner image formed on the image bearing body onto the intermediate transfer body is repeated four cycles, and the toner images of four colors transferred on the intermediate transfer body are collectively transferred onto a transfer member such as a sheet.
In the structure adopting the four-rotation intermediate transfer system as stated above, the plural color developing devices are changed over at the time of formation of the respective color images and the changed one comes in contact with the one photoconductor. Accordingly, when the development of the first color is ended at the time of image formation of the first color, the developing device of the second color immediately comes in contact with the photoconductor, and the development of the second color is started. On the other hand, the transfer of the first color is ended and when the residual transfer toner reaches the next development portion, the developing device of the second color is in contact, and when development simultaneous cleaning is performed in the cleanerless process, the residual transfer toner of the first color is mixed into the developing device of the second color, and there arises a problem of color mixture.
For example, at the time of transfer of a yellow toner image from the photoconductor to the intermediate transfer body, yellow transfer residual toner is generated on the photoconductor. However, in the four-rotation type color image forming apparatus, before the yellow transfer residual toner is sent to the development position by the rotation of the photoconductor, the developing device is changed from the yellow developing device to the next magenta developing device by the rotation of a rotator. Thus, the yellow transfer residual toner on the photoconductor is collected into the magenta developing device, and there occurs a disadvantage that the magenta toner and the yellow toner are mixed.
In order to eliminate this disadvantage, there is proposed a method of using a collection apparatus in which plural collection units to collect transfer residual toner are disposed around a photoconductor, or plural collection units are substantially cylindrically arranged together on a rotation member, and the collection units are sequentially made opposite to the photoconductor by the rotation of the rotation member to collect the residual toner (see, for example, JP-A-9-23696).
Besides, in a four-rotation intermediate transfer system image forming apparatus, there is disclosed a technique in which development rollers coming in contact with a photoconductor and collection units are included, and collected toners are returned to the developing devices of the same colors (see, for example, U.S. Pat. No. 6,625,414B2). In these techniques, as the collection of the transfer residual toner by the collection unit proceeds, the transfer residual toner is accumulated in the collection unit. Since the amount of toner which can be collected by the collection unit is restricted to a certain degree by the restriction of an arrangement space and the like, there is a fear that in a state where the amount of transfer residual toner becomes very large, the amount of transfer residual toner to be collected exceeds the collection capacity of the collection unit. In the case where the amount of transfer residual toner exceeds the collection capacity of the collection unit, the transfer residual toner which could not be collected remains on the photoconductor, and the color mixture of toners is caused.